1. Field of the Invention
The present invention relates to the field of systems and methods for communication wherein some degree of adaptive link control is provided.
2. Description of the Related Technology
The use of multiple-antenna transmission techniques can drastically improve the performance of wireless communication systems. More specifically, such techniques can be used to increase antenna gain and directionality (e.g., beamforming), to improve link robustness (e.g., space-time coding) or to improve spectrum efficiency (e.g. space division multiplexing).
Techniques where multiple antennas are considered both at transmit and receive sides can combine those assets and are referred to as MIMO (multiple-input, multiple-output). On the other hand, because of its robustness in harsh frequency selective channel combined with a low implementation cost, orthogonal frequency division multiplexing (OFDM) is now pervasive in broadband wireless communication. Therefore, MIMO-OFDM schemes turn out to be excellent candidates for next generation broadband wireless standards (like e.g. IEEE802.11n).
Traditionally, the benefit of MIMO schemes is characterized in terms of multiplexing gain (i.e. the increase in spectrum efficiency) and diversity gain (namely, the increase in channel variation immunity, quantified as the order of the bit error rate decay as a function of the signal-to-noise ratio (SNR)). Given a multiple-input, multiple-output (MIMO) channel and assuming a high SNR, there exists a fundamental trade-off between how much of these gains a given coding scheme can extract. The merit of a new multiple-antenna scheme is then mostly evaluated with regard to that trade-off. However, due to the recent trend towards having broadband wireless support in small form-factor, potentially multi-mode, battery-powered devices, such as personal digital assistants (PDAs) and smartphones, the energy efficiency is an increasingly important aspect to take into account when assessing a new scheme. Characterizing how diversity gain, multiplexing gain and/or coding gain influence the user-relevant trade-off between transmission rate and energy efficiency trade-off is not trivial.
Transceivers' power consumption is generally speaking made of two terms. The first term corresponds to the power amplifier and depends on the transmit power, hence on the link budget. The second term corresponds to the other electronics power consumption and is independent of the link budget. This is further referred to as dynamic and static power consumption, respectively. The impact of multiple-antenna transmission (MIMO), when compared with traditional single-antenna (SISO) transmission, is two-fold. On the one hand, the general benefit in spectral efficiency versus SNR can be exploited either to reduce the required transmit power, with impact on the dynamic power consumption, or to reduce the transceiver duty cycle with impact on both dynamic and static power contributions. On the other hand, the presence of multiple antennas requires duplicating a part of the transceiver circuitry, which increases both the static and dynamic terms. The question whether multiple-antenna techniques increase or decrease the energy efficiency in this context has only very recently been addressed in the literature.
Based on comprehensive first order energy and performance models of sensor-targeted transceivers, taking both static and dynamic power into account, the energy efficiency of single-carrier Space-Time Block Coded (STBC) MIMO links versus traditional single antenna (SISO) links has been evaluated. Interestingly, it is shown that in short-/middle-range applications such as sensor networks—and by extension, WLAN—non-adaptive multiple-antenna techniques actually degrade the energy efficiency at a same rate. However, when combined with adaptive modulation in so-called adaptive multiple-antenna techniques, the energy-efficiency can be improved. Energy-efficiency can further be improved by adaptively combining multiplexing and diversity. Adaptivity is hence mandatory to achieve high-performance and energy-efficient transmission.
Adaptive MIMO-OFDM schemes have also been studied in the context of broadband communication. A scheme is proposed to switch between diversity and multiplexing codes based on limited channel state information (CSI) feedback. Adaptation is carried out to minimize the bit error probability (BER). Pragmatic coarse grain adaptation schemes have been evaluated. Modulation, forward error correction (FEC) coding rate and MIMO encoding are adaptable according to a CSI estimator—specifically, the average signal-to-noise ratio (SNR) and packet error rate (PER)—in order to maximize the effective throughput. More recently, fine grain adaptation schemes, tuning carrier-per-carrier the modulation and MIMO encoding, have been proposed. The main challenge with such schemes is to provide the required CSI to the transmitter with minimal overhead. Yet, in none of the prior art contributions energy-efficiency is considered. More specifically, the static power consumption is never considered in the optimization although it can be shown to be of great importance. Moreover, adaptation policies have been designed to maximize gross data rate and/or minimize (uncoded) bit error rate without taking into account the medium access control (MAC) aspects that have been shown to be of significant importance both for the net throughput and for the energy efficiency.